Linker error with global variables in .h files

Question

I have the following .h file

#ifndef _EXCEPTION_H
#define _EXCEPTION_H

...

const char* _ErrorCode[] = {
    "%s",
    "Error: Index is out of range.",
    "Error: Stack is empty.",
    "Error: New matrix size is not greater than current one.",
    "Error: Index or size cannot be negative.",
    "Error: Method option is not within permissible options."
    "Error: File could not be found or created."
    "Error: There was not enough memory to allocate container."
                       }; // Error code list

...

#endif

that is included by many files. But when i try to compile, i get the following linker error:

4 warnings generated.
duplicate symbol __ErrorCode in:
    exception.o
    graph.o
duplicate symbol __ErrorCode in:
    exception.o
    io.o
duplicate symbol __ErrorCode in:
    exception.o
    list.o
duplicate symbol __ErrorCode in:
    exception.o
    matrix.o
duplicate symbol __ErrorCode in:
    exception.o
    stack.o

which means the compiler is redefining the _ErrorCode global variable in all the files in which exception.h is included even though there is an #ifndef at the beginning of the file. How do i stop this from happening? If i can't, how do i define a read-only array of strings that i can print in case of errors like this?

Answer 1

If you're using #include "exception.h" in exception.c, graph.c, io.c, and every other C source file in your project, then that's the equivalent of copying the contents of exception.h into the top of every C source file.

That means that in each C source file, there's an _ErrorCode variable defined. When the object files from each C source file are linked, since __ErrorCode is defined in each object file, that's a naming conflict.

The solution is to use extern as Dabo suggests - if you use extern const char* __ErrorCode[]; in the header file, then each object file will know to expect an __ErrorCode variable defined elsewhere. Then, you can define the variable in exception.c. During linking, the extern variables will all be resolved to the __ErrorCode defined in exception.c.

Answer 2

From what i understand the linker doesn't like initializing global variables in .h files because it considers them as re-defined in every instance of the file. The solution is to make

extern const char* _ErrCode[];

then on the .c do

const char* _ErrCode[] = {...}